1. Field of the Invention
The present invention relates to a dielectric ceramic material. More particularly, the present invention relates to a dielectric resonator comprising a perovskite type compound having a novel chemical composition, which is especially valuable as a dielectric resonator to be used in the microwave region.
2. Description of the Prior Art
Recently, dielectric ceramic materials are widely used in the microwave region with practical application of car telephones and personal wireless installations, conversion of microwave circuits to integrated circuits and application to gun oscillators. These microwave dielectric ceramics are mainly used for resonators, and the properties required for resonators are as follows. Namely, (1) since the wavelength is shortened to 1/.sqroot..epsilon. (.epsilon. stands for the dielectric constant), the dielectric constant should be large so as to satisfy the requirement of reduction of the size, (2) the dielectric loss at a high frequency should be small, and (3) the change of the resonance frequency according to the temperature, that is, the temperature dependency of the dielectric constant, should be small and stable.
As the known dielectric ceramic material of this type, there can be mentioned a material of the BaO-TiO.sub.2 system, a material of the REO-BAO-TiO.sub.2 system (RE stands for a rare earth element) and a material of the (BaSrCa)(ZrTi)O.sub.3 system.
The material of the BaO-TiO.sub.2 system has a dielectric constant .epsilon.r is as high as 38 to 40 and the dielectric loss tan .delta. is less than 2.00.times.10.sup.-4, but a single phase material having a temperature coefficient .tau.f of 0 can hardly be obtained and the changes of the dielectric constant and the temperature dependency of the dielectric constant according to the change of the composition are great. Accordingly, it is difficult to control the temperature coefficient (.tau.f) of the resonance frequency at a low level stably while maintaining a high dielectric constant and a low dielectric loss. In case of the material of the REO-BaO-TiO.sub.2 system, the dielectric constant .epsilon.r is very high and 40 to 60 and a product in which the temperature coefficient .tau.f of the resonance frequency is 0 can be obtained, but the dielectric loss tan .delta. is large and exceeds 5.0.times.10.sup.-4. In the material of the (BaSrCa)(ZrTi)O.sub.3 system, the dielectric loss tan .delta. is small and less than 2.00.times.10.sup.-4 and the dielectric constant .epsilon.r is appropriate, but the deviation of the temperature coefficient .tau.f of the resonance frequency is great and it is difficult to obtain products having constant properties stably. Incidentally, the dielectric loss tan .delta. referred to in the instant specification is the value measured at 500 to 600 MHz by using a di-entrant resonator. As is seen from the foregoing description, a composition capable of completely satisfying the above-mentioned requirements for the microwave dielectric material has not been developed.